Axon Self-Destruction: New Links among SARM1, MAPKs, and NAD+ Metabolism

Abstract

Wallerian axon degeneration is a form of programmed subcellular death that promotes axon breakdown in disease and injury. Active degeneration requires SARM1 and MAP kinases, including DLK, while the NAD+ synthetic enzyme NMNAT2 prevents degeneration. New studies reveal that these pathways cooperate in a locally mediated axon destruction program, with NAD+ metabolism playing a central role. Here, we review the biology of Wallerian-type axon degeneration and discuss the most recent findings, with special emphasis on critical signaling events and their potential as therapeutic targets for axonopathy.

Figure 1

Time course of events in axon degeneration in cultured DRG neurons. The “latent period” (from injury to ~4–6 hours) precedes morphological changes and is characterized by NMNAT2 depletion (Di Stefano et al., 2015), and transient phosphorylation of MKK4 at S257/T261 (Yang et al., 2015). During this window, axon degeneration can be halted by several protective manipulations including NMNAT protein transduction (Sasaki and Milbrandt, 2010), SARM1 cleavage (Gerdts et al., 2015), and addition of FK866 (Sasaki et al., 2009b; Di Stefano et al., 2015) or JNK inhibitors (Miller et al., 2009). As NAD+ declines from 3–6 hours (Wang et al., 2005), ATP levels also decline (Yang et al., 2015), and an active and irreversible phase of axon degeneration begins with neurofilament (NF) proteolysis (Yang et al., 2013) stimulated by calcium influx and finally frank morphologic fragmentation of the axons. These events also occur in vivo, albeit over a slower time course.

Figure 2

Pathways of NAD+ synthesis and breakdown. TOP: NAD+ is synthesized from nicotinamide (Nam), nicotinic acid (Na), nicotinamide riboside (NR), or tryptophan (Trp). All synthetic pathways require NMNAT. Nicotinic acid mononucleotide (NaMN) can be synthesized from nicotinamide mononucleotide (NMN) in some bacteria by the enzyme NMN deaminase (NMNd), which has no mammalian ortholog. NAD+ is broken down by multiple classes of enzymes including the glycohydrolase CD38 (Aksoy et al., 2006), poly-ADP-ribose polymerases (PARPs), NUDIX phosphohydrolases (McLennan, 2006), ADP ribosyltransferases (ARTs), and sirtuins (SIRTs). PARPs create polymers of ADPR that are usually attached to a protein substrate. ARTs transfer adenosine diphosphate ribose (ADPR) from NAD+ to an acceptor molecule (X) such as a protein. SIRTs transfer an O-acetyl group from a protein substrate to the ADPR moiety of NAD+ to yield O-acetyl-ADPR and Nam. NaMN=nicotinic acid mononucleotide; NaAD=nicotinic acid adenine dinucleotide; NMN=nicotinamide mononucleotide; NAD+=nicotinamide adenine dinucleotide; AMP=adenosine monophosphate; NAPRT=nicotinic acid phosphoribosyltransferase, NMNAT=nicotinamide mononucleotide adenyltransferase, NADSYN=NAD synthetase, NAMPT=nicotinamide phosphoribosyltransferase; NRK=nicotinamide riboside kinase, NUDIX=nucleoside diphosphate moiety linked X. BOTTOM: Structure of NAD+ with substrate moieties approximately outlined. For a more detailed overview of this pathway, please see these reviews: (Belenky et al., 2007; Chiarugi et al., 2012).

Figure 3

A working model of SARM1 auto-inhibition and activation upon injury. TOP: SARM1 is made up of three regions: 1) an auto-inhibitory N terminus (Nterm) comprised of multiple armadillo repeat motifs, 2) tandem SAM domains that mediate SARM1-SARM1 binding (SAMx2), and 3) a TIR domain that triggers axon degeneration upon multimerization. BOTTOM: SARM1 multimers are inactive (auto-inhibited) in uninjured axons. Injury leads to SARM1 activation, perhaps through release of inhibition, exposing TIR domain multimers that transmit a pro-destructive signal to unknown effector molecule(s).

Figure 4

Working model of an integrated axon degeneration signaling cascade. Injury leads to SARM1 activation (Osterloh et al., 2012) and NMNAT2 depletion (Gilley and Coleman, 2010). PHR1 promotes NMNAT2 turnover, leading to faster depletion (Babetto et al., 2013; Xiong et al., 2012). Activated SARM1 promotes NAD+ depletion (Gerdts et al., 2015) and NMNAT2 loss prevents NAD+ synthesis and causes an increased NMN to ATP+ADP+AMP ratio, which may activate SARM1 (Gilley et al., 2015). NAD+ loss leads to glycolytic failure and ATP depletion. SARM1 also activates MAPK pathway signaling (Yang et al., 2015), which promotes SCG10 proteolysis (Shin et al., 2012a) and contributes to ATP depletion, perhaps via NAD+ depletion. MAPK activation is counteracted by injury-stimulated ATK1 activity (Yang et al., 2015), and AKT is in turn destabilized by ZNRF1 (Wakatsuki et al., 2011). Energetic failure promotes ionic imbalance including intraaxonal calcium accumulation, leading to calpain activation and proteolysis of intermediate filaments in the axonal cytoskeleton (Yang et al., 2013). Cumulative structural damage leads to irreversible fragmentation of the damaged axon (Wang et al., 2012). Arrows with questions marks (?) reflect postulated interactions.